1. Field of the Invention
The present invention relates to a band brake used for an automatic transmission of a vehicle and the like.
2. Description of the Related Art
In an automatic transmission of an automobile and the like, a band brake, which is adapted to transmit a drive force or interrupt transmission of the drive force, is larger in static torque capacity than a multi-board clutch which is equal in size to the band brake. On the other hand, there has been a strong demand for miniaturization of an automatic transmission of an automobile and the like, and therefore the recent tendency is to increase the application of a band brake to an automatic transmission of an automobile and the like. In addition, there has been a strong demand for miniaturization of the band brake.
In the band brake, the brake band is abutted against the outer cylindrical surface of a drum to transmit the drive force or interrupt the transmission of same. In association with miniaturization of the automatic transmission, the band brake is also miniaturized. Hence, a method of abutting the brake band against the outer cylindrical surface of the drum is restricted.
FIG. 1 shows the general arrangement of a band brake of the type which is popularly employed. The band brake, indicated generally by reference number is formed by a transmission case 1, a drum 2 rotatably held in the case 1, a C-shaped brake band 3, and an anchor pin 4 and an apply pin 5 which are held in the case 1. The transmission case 1 has a space which accommodates the drum 2 and the brake band 3.
The brake band 3 includes a C-shaped metal band 35, a frictional lining 36 bonded to the inner cylindrical surface of the metal band 35; and an anchor bracket 31 and an apply bracket 32 which are secured to both ends of the band 35. The anchor bracket 31 has a protrusion 33 which is extended in the radial direction of the drum 2, and the apply bracket 32 has a protrusion 34 which is also extended in the radial direction of the drum 2.
The anchor pin 4 is a cylindrical member and is laid in parallel with the rotary axis of the drum (i.e., in the direction perpendicular to the surface of the drawing), and is in contact with the protrusion 33 of the anchor bracket 31.
The apply pin 5 is a cylindrical member a spherical end portion, which is slidably held by the transmission case 1 in a direction substantially tangential to the drum 2. The end portion of the apply pin 5 pushes and drives the apply bracket 32, to there by press the brake band 3 against the outer cylindrical surface of the drum 2 or to retract the brake band 3 to release the drum 2.
It is difficult to make the dimensions of all the components of the band brake correct. Thus, the dimension of the components and the contact of the anchor pin against the anchor bracket are unavoidably fluctuated. Accordingly, an excessively high load is locally applied thereto. In order to overcome this difficulty, a modified band brake has been proposed. That is, as is seen from FIG. 4 which is a side view of a part of the anchor bracket, FIG. 5 which is a plan view as viewed in the direction of the arrow S in FIG. 4, and FIG. 6 which is an enlarged diagram of a part of FIG. 5, in the band brake, the protrusion 33 has an engaging protrusion 31a on the anchor pin side in such a manner that the engaging protrusion 31a is in point contact with the cylindrical anchor pin 4. As shown in FIG. 6, on the side surface of the protrusion 33 (on the anchor pin side) of the protrusion, the engaging protrusion 31a, which is arcuate in section and has a radius indicated by the arrow R, is arranged at the middle of the anchor bracket as viewed in the direction of width thereof.
For example, the conventional band brake in which an anchor pin 4 shown in FIG. 1 is in point contact with a protrusion 33 of an anchor bracket 31 has an effect of dispersing a load uniformly. However, it is disadvantageous in that, because of the point contact, the contact region is greatly worn, which affects the drive stroke of the apply pin 5, which adversely affects the tightening response of the brake band. That is, the conventional band brake is low in operational stability. Furthermore, in the engaging region of the anchor pin 4 and the anchor bracket 31, there is nothing which directly regulates the brake band in the axial direction, so that the brake band is unstable in its position. Those difficulties are significant especially when the brake band is not tightened, and therefore, drag torque is increased, and the frictional lining is quickly worn out.